A well-known toggle bolt assembly consists of a threaded bolt on which is mounted a threaded toggle head including a pair of spring-loaded wings, each of which can be folded to a position parallel to the bolt, thereby permitting the toggle head to be passed through a hole in the surface on which an attachment is to be made. After passing through the hole, the spring-loaded wings spring outwardly away from the bolt to a position approximately perpendicular to the bolt. The total length of the extended wings is greater than the diameter of the hole through which the folded toggle head was inserted, so that the toggle assembly cannot be pulled out of the hole. By rotating the bolt on which the toggle head is mounted, a firm attachment of a desired object, previously positioned between the toggle head and the head of the bolt, can be made to the surface. The relatively complex construction renders toggle bolt assemblies of the type described expensive and also limited in load carrying capacity because of a tendency to fail at the articulated joints which are present therein.
A toggle bolt assembly in which the toggle head is of solid construction, without the joint necessary in the folding wing type, would be not only stronger, but more reliable and less costly than the folded wing construction. The design of such an assembly, however, presents obvious difficulties. Although an elongated head can be passed axially through the opening in which the fastener is to be installed, once through the hole it must be reoriented in a transverse position spanning the hole and in position to engage the threads of the bolt with which it is to be used. The problem is, of course, aggravated by the fact that access to the rear surface of the member in which the fastener is to be installed is generally not available.
Several toggle bolt assemblies employing a solid, i.e., non-articulated, head are known, all of which have some shortcomings.
In Italian Pat. No. 493,279, granted July 23, 1953, the end of the bolt is provided with a hook on which the solid toggle head is suspended. A flexible braid attached to the head and extending through the hole in the workpiece is used to pull the head into contact with the threads on the bolt in order to permit engagement of the bolt with the head. The need for manipulation of the braid in order to secure engagement between the bolt and the nut, and the possibility that the braid might break or become fouled with the workpiece, represent disadvantages of this type of construction.
In U.S. Pat. No. 3,175,452, there is proposed a flexible extension on the end of the bolt on which an elongated nut or toggle head is positioned during installation of the assembly. The extension is bent at approximately right angles to permit the elongated head or nut to pass through an opening in the workpiece. After passing through the opening, the inherent resiliency of the extension positions the toggle head across the opening. The extension is in the form of a thin strip having the same diameter as that of the bolt and threaded along its edges with threads corresponding to those on the bolt. One problem encountered with this type of construction lies in the difficulty encountered in fastening the thin end of the extension, typically formed of a resilient plastic material, to the end of the bolt. In addition, the small area of contact between the threads of the toggle head and the thin edge of the flexible extension present the distinct possibility that the head will be pulled off the strip if too great a force is exerted on the bolt during the initial stages of tightening.
British Pat. No. 1,045,984, published Oct. 19, 1966, discloses a construction similar to that of U.S. Pat. No. 3,175,452, in which there is employed a thin extension which is threaded on its narrow edges. The British patent provides additional full circular sections at both ends of the narrow extension which overcome some of the disadvantages of the construction shown in U.S. Pat. No. 3,175,452. The construction shown in the British patent, however, requires an auxiliary threaded extension or bore at the end of the main bolt and a corresponding threaded hole or projection on the flexible extension, a complex construction which obviously increases the cost of the assembly.
Swedish Pat. No. 216,380, granted May 11, 1967, discloses a similar form of toggle employing a thin flexible member threaded on its narrow edges. The patent recognizes the problem of insuring a smooth transition from the threads of the flexible extension to those of the threaded shaft by providing a full circular section intermediate the threaded shaft and the flexible member, the circular section having undercut flexible threads which deform sufficiently under stress to permit engagement of a threaded head with the shaft, even if the respective threads do not exactly match. The complex construction of the transition zone of the fastener increases the cost of manufacturing the assembly.
British Pat. No. 606,902, accepted Aug. 23, 1948, discloses a fastener employing a solid internally threaded head mounted on a helical coil spring which can be bent to permit insertion of the head through a mounting hole. The coil serves as the only means for tightening the fastener, so that the problem of matching the threads to permit a smooth passage of the head from the coil spring to a rigid threaded shaft is not encountered. By the same token, the relatively weak spring used as the sole support member limits the load-carrying capacity of the assembly.